1. Field of the Invention
This invention relates to an original and original size detecting device in an image forming apparatus.
2. Description of the Related Art
In a conventional image forming apparatus, such as shown in FIGS. 1 and 8, in the detection of the size of an original is performed by a reflection type sensor 10 mounted on a first mirror frame 12 so that the reflection type sensor 10 moves with the first mirror frame 12. In the reflection type sensor 10, light emitting means, namely, a light emitting element 1 and detecting means constituted by photo-transistor, namely, a light receiving element 2 are provided adjacent to each other. To detect the size of an original placed on a contact glass plate 26, the first mirror frame 12 is moved from the side of one end of the original to the side of the other end while an original retainer 11 is open. During this movement, the light emitting element 1 of the reflection type sensor 10, mounted on the first mirror frame 12, emits light to irradiate the original. The light receiving element 2, moving together with the light emitting element 1, receives light reflected from the original. When the light emitting element 2 is moved to the other end of the original where the light reflected from the original can no longer be detected, the first mirror frame 12 is stopped. Thus, the size of the original is detected from the distance the first mirror frame 12 has moved.
As was described above, while the reflection type sensor 10 is being moved, the light emitting element 1 emits light based on instructions from control means 57, and the light receiving element 2 receives light reflected from the original. As shown in FIG. 8 , the reflection type sensor 10 is provided with determining means 3 for determining the presence or absence of an original. The determining means 3 compares the output signal from the light receiving element 2, a detection value, with a reference value which has been set by a reference value setting means 5. When the determining means 3 determines from the comparison that the light reflected from the original is no longer being received, the distance the reflection type sensor 10 has moved is measured to determine the size of the original by control means 57.
In the conventional original size detecting device, the size of an original is detected with the original retainer 11 open. Hence, when the light emitting element 1 emits light and the light receiving element 2 receives light reflected from the original, external light, such as light from a florescent lamp or sunlight, may be received by the light receiving element 2 increasing the detection output of the light receiving element 2. Thus, even when the light receiving element 2 has been moved to the position where the original is not present (i.e., the original is not covering that part of cover glass plate 26 where the reflected type sensor 10 is currently positioned), the presence of an the original will be erroneously determined. The erroneous determination occurs because the external light causes the light receiving element 2 to produce a detection value which when compared with the predetermined reference value set by the reference value setting means 5 indicates that the original is present. As a result, the detected size of the original is not correct.